1. Field of the Invention
This invention generally relates to systems and methods for inspecting wafers. Certain embodiments relate to a system that includes a detection subsystem configured to separately and simultaneously detect light scattered from different portions of a single TO spot obliquely illuminated on a wafer and to separately generate output responsive to the separately detected light that can be used to detect defects on the wafer.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices. Accordingly, much work has been done in the field of wafer inspection to increase the sensitivity of inspection systems to smaller and smaller defects.
However, in some cases, unpatterned wafer inspection has reached a limitation in sensitivity, both on ultra smooth surfaces and on rough surfaces. For example, as defect sizes decrease, the difficulty of detecting relatively small defects on relatively rough wafer surfaces increases. Previously, the scattering of light from relatively rough surfaces did not substantially limit inspection system performance since the defects being detected were relatively large. However, as the size of defects decreases, the amount of light scattered from the defects also decreases. As such, the amount of light scattered from defects of relatively small size may be much closer to the amount of light scattered from relatively rough surfaces thereby reducing the sensitivity of many systems for inspection of such surfaces. Therefore, although many currently available inspection systems are capable of detecting relatively large defects on relatively rough surfaces and/or relatively small defects on relatively smooth surfaces, there is still a need for an inspection system that can detect relatively small defects on relatively rough surfaces as well as even smaller defects on relatively smooth surfaces.
Many inspection systems such as those described above are configured to image a single spot or line on the wafer at normal and/or oblique angles of incidence using spherical and/or cylindrical lenses. The single spot or line imaging of these systems also contributes, at least in part, to the relatively low sensitivity (e.g., relatively low signal-to-noise ratio, SNR) of the systems when inspecting certain surfaces. In particular, since a single spot or line on the wafer plane is relatively large (particularly in comparison to the size of the defects typically being detected), the light scattered from the illuminated spot or line will contain a relatively large amount of scattering from the surface of the wafer. Such scattering may be relatively low for relatively smooth surfaces. However, the scattered light from relatively rough wafer surfaces may be much higher and will, therefore, adversely affect the sensitivity of the inspection system.
One way to increase the SNR for relatively rough surface inspection is to decrease the size of the spot on the wafer. However, decreasing the size of the optical spot on the wafer often undesirably decreases the throughput of the inspection system.
Accordingly, it would be advantageous to develop systems and methods for inspecting wafers with enhanced sensitivity for detecting relatively small defects on both smooth and rough surfaces while maintaining a relatively high throughput.